There are many types of surgically implantable prostheses available as substitute for natural bone and joints which have become damaged by disease or trauma. Practitioners have had high success in substituting by excising portions of the natural bone or joint and replacing such with a prosthesis.
A problem exists with presently available prostheses in the case of juveniles. As the patient grows, the limb having the prosthesis implanted typically does not grow at the same rate as the opposing limb. For example, it is not uncommon to find a patient having a prosthesis in a leg to have one leg shorter than the other.
In order to overcome the above-mentioned problem, adjustable length prosthetic joint implants have been developed. An example of such an implant is disclosed in U.S. patent application No. 4,502,160 to Moore, et al., issued Mar. 5, 1985 and assigned to the assignee of the present invention. The device disclosed in the Moore, et al. patent utilizes a threaded stem which can be extended from a barrel by a threaded nut mechanism. The stem is extended by exposure of a ratchet mechanism on the nut member which is rotated by a gear-carrying key. Use of such a system requires an incision of approximately two inches long which must be made each time an adjustment is desired. In other words, the use of such a device is invasive because each time an adjustment is made, access to the nut by the key member must be achieved. Since it is advisable for invasive surgery to be kept to a minimum, and a general anesthetic is necessary for such surgery for the adjustment of the length of the prosthesis, practitioners tend to overshoot the necessary extension at the time the post-operative expansion surgery is made to compensate for future growth. The extension causes stretching of ligaments and muscles to the extent that extensive rehabilitation is needed for the return of full range of motion of a joint. Further, since such devices are generally made out of titanium or titanium-alloy, and titanium surface wear properties are poor, the threaded portions of the device are susceptible to wear.
In view of the above problems, it is desirable to provide an adjustable length prosthesis which does not require an invasive element. That is, it is desirable to eliminate the post-operative surgery for expansion of the device. Additionally, it is desirable to eliminate the threads of the device. The present invention provides an adjustable length prosthetic implant requiring minimal or no post-operative surgery for expansion and does not require the use of a threaded stem element.